Amazon has introduced an experimental new payment model that would see authors paid not for every book sold, but for every page read of that book. Don't worry, Amazon isn't watching you read that paperback you ordered — it's just for authors using the Kindle Unlimited and Lending Library self-publishing services, which provide books on demand for subscribers.
It works like this: There's a "fund" available for authors who publish through Amazon, based on the number of paying subscribers. At the end of each month, Amazon looks at how many pages were read total, and how many pages of those belonged to a certain author — and they're given a share of the total amount based on that.
So if everyone who started your 150-page book finished it, you'd earn the same as your friend, who wrote a 300-age book that everyone stops reading halfway through.
It's a bit like the streaming model used by Spotify and the like, where artists are paid per listen rather than by albums sold. It hasn't worked out well for musicians, but the approach might work better for authors — assuming they don't start multiplying word counts and slotting in lots of illustrations to inflate page numbers. The new payment method will start on July 1.